With the trend towards high functionalization and miniaturization of electronic devices, the passive devices in the electronic systems have increasingly predominated. At present, a majority of passive devices are present in the form of a discrete element surface mounted on a circuit board substrate and occupy about 80% by area of the circuit board substrate surface, for example, the passive devices are 20 times as many as the active devices in a cellular phone. The surface mounted passive devices also require many interconnections and welded dots on a surface of the substrate so that the electric properties and reliability of the materials and system are greatly declined.
In order to provide an electronic system which has lower price, lighter weight, better properties and higher reliability, it is effective to convert the previous surface-mounted passive devices into the embedded passive devices. By using a multilayered printed circuit board (PCB) technology, the embedded passive devices are produced by including passive devices in the layers of the substrate, which are capable of saving the area of the surface while reducing impedance and improving reliability and electric properties. Therefore, the embedded passive devices have potential of solving the above problems.
Among all of passive devices, the capacitor has particularly attracted attention due to the high number of surface capacitors typically used in a device. The embedded capacitor can contribute to the reduction of size of a final product comprising the capacitor and the improvement of properties of the product.
U.S. Pat. No. 5,162,977 discloses a dielectric material which has a dielectric constant 10 times higher than that of epoxy resin and is produced by impregnating glass fiber fabric with an epoxy resin and ceramic powders. However, the capacitance density of the dielectric material is not sufficiently high, mainly because the glass fiber fabric itself has low dielectric constant and the thickness of the dielectric layer can not be easily controlled by this impregnation process so that its capacitance density fluctuated greatly.
WO 03/011589 discloses a thin capacitor which has a three-layer structure, that is, thermoset resin layer, heat-resistant thin film layer and thermoset resin layer, and possesses excellent mechanical strength capable of satisfying PCB double-side etching process requirement. However, the heat-resistant thin film in the thin capacitor contains excessive halogen and can not satisfy the halogen-free requirement of the current electronic products.
WO 2010/127245 A2 discloses a thin dielectric material reinforced with a liquid crystal polymer nonwoven fabric, which can achieve relatively high capacitance density and mechanical strength, but the liquid crystal polymer nonwoven fabric can not be stably supplied and has a prohibitive price which is not acceptable for common customers.
Therefore, there is a need to develop a dielectric material which possesses relatively high capacitance density and mechanical strength meanwhile meets the halogen-free requirement for electronic products, consequently this dielectric material can be used to manufacture capacitors and other electronic products.